I thought that the radiation ceased after the bomb’s explosion was completed.
The daughter products of the nuclear reaction of the bomb and the immediate target material have various half-lives. This material is raised aloft and falls to earth in a plume shaped by the wind. Downwind of the bomb, the amount of radiation will be such that people could receive a fatal dose for some days afterwards.
see for example:
All about fallout:
https://archive.org/details/AboutFal1963
The radiation from the bomb itself ceases after the fission/fusion event, yes. However, the neutron radiation in turn can make *other* objects radioactive for a lot longer through the phenomenon of neutron activation. Copying wikipedia because I don’t have the time to type it out myself: http://en.wikipedia.org/wiki/Neutron_bomb#Effects_of_a_neutron_bomb_in_the_open_.26_in_a_city
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Upon detonation, a 1 kiloton neutron bomb near the ground, in an airburst would produce a large blast wave, and a powerful pulse of both thermal radiation and ionizing radiation, mostly in the form of fast (14.1 MeV) neutrons. The thermal pulse would cause third degree burns to unprotected skin out to approximately 500 meters. The blast would create at least 4.6 PSI out to a radius of 600 meters, which would severely damage all non-reinforced concrete structures, at the conventional effective combat range against modern main battle tanks and armored personnel carriers (<690–900 m) the blast from a 1 kt neutron bomb will destroy or damage to the point of non-usability almost all un-reinforced civilian building. Thus the use of neutron bombs to stop an enemy armored attack by rapidly incapacitating the crew with a dose of 8000+ Rads of radiation,[38] which would require exploding large numbers of them to blanket the enemy forces, would also destroy all normal civilian buildings in the same immediate area ~600 meters,[38][39] and via neutron activation it would make many building materials in the city radioactive, such as Zinc coated steel/galvanized steel(see Area denial use below). Although at this ~600 meter distance the 4-5 PSI blast overpressure would cause very few direct casualties as the human body is resistant to sheer overpressure, the powerful winds produced by this overpressure are capable of throwing human bodies into objects or throwing objects-including window glass at high velocity, both with potentially lethal results, rendering casualties highly dependent on surroundings, including on if the building they are in collapses.[40] The pulse of neutron radiation would cause immediate and permanent incapacitation to unprotected outdoor humans in the open out to 900 meters,[4] with death occurring in one or two days. The lethal dose(LD50) of 600 Rads would extend to about 1350–1400 meters for those unprotected and outdoors,[38] where approximately half of those exposed would die of radiation sickness after several weeks.
However a human residing within, or is simply shielded by at least 1 of the aforementioned concrete buildings with walls and ceilings 30 centimeters/12 inches thick, or alternatively of damp soil 24 inches thick, the neutron radiation exposure would be reduced by a factor of 10.[41][42]
Furthermore the neutron absorption spectra of air is disputed by some authorities and depends in part on absorption by hydrogen from water vapor. It therefore might vary exponentially with humidity, making neutron bombs immensely more deadly in desert climates than in humid ones.[38]
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